Solid state image sensing has received much attention as an effective means for inputting an image to a video camera or image processing device. Solid state image sensing devices are categorized as a CCD type, MOS type or another type using electron transfer forms.
The conventional solid state image sensing device has a plurality of vertical charge transfer channels, an array of photodiodes, a horizontal transfer channel, and a group of electrodes. The electrodes include a first polysilicon electrode for a transfer gate and second and third electrodes for the vertical transfer channel. The device has a plurality of cells which are formed by these elements, and has thick silicon oxide layers formed by LOCOS for cell isolation.
Electrons stored in the photodiode are transferred to vertical transfer channel by way of a transfer gate. The electrons transferred to the vertical channel are transferred to the horizontal transfer channel and an image processing means sequentially.
In such conventional solid state image sensing device, it is difficult to reduce the cell size because field oxide layer uses a large area. Furthermore, such a device has low reliability because many steps are necessary to manufacture such a structure.
An early improvement was made by using a solid state image sensing device with two polysilicon layers.
Such a device as shown in FIG. 8(A) and (B) has a plurality of photodiodes 11, a first area 13 of N type semiconductor serving as an electron transfer channel, first and second polysilicon layers 15, 17 acting as transfer gates to a horizontal transfer channel, which is formed on a p well area 23 in a silicon substrate of N type. The layer 17 unites the transfer gate.
In this improved device, the second polysilicon layer 17 is combined with transfer gates in a body which emits electrons from photodiode 11 to the first area 13. Three different voltages are applied to the second polysilicon layer, and two different voltages are applied to the first polysilicon layer. The highest of the three voltages applied to the second polysilicon layer is used for reading out electrons moving from photodiode 11 to the first areas 13. The other voltages are applied to the first and second polysilicon layers and are used for transferring electrons in the first area 13 to the horizontal channel. In this case, since one or both of the two different voltages for transferring electrons to the horizontal channel are present for negative potential, the adjacent cells located in the vertical direction are not separate field oxide layers but polysilicon layers. Therefore, the field oxide areas on the surface of the substrate are reduced in number and total area.
Since the conventional device as shown in FIG. 8 (A) and (B) has the second polysilicon layer which combine with the transfer gates in a body for reading out electrons from the photodiode, it is necessary to form a field oxide layer between the adjacent cells located in the horizontal direction for separating the cells electrically. Therefore, the field oxide layer between the cells is not removed completely.
It is possible to use a third polysilicon layer instead of the field oxide layer for separating the cells electrically. But such a device has a new problem that the number steps are increased and it is difficult to reduce the cell size.
Accordingly, the general object of this invention is to provide a solid state image sensing device capable of separating cells electrically without using a field oxide layer.
Another object of this invention is to provide a solid state image sensing device having cells which are separated electrically from one another by two polysilicon layers.